823 research outputs found
The influence of mesoscale eddies and topology on southern ocean flow
No full textThe physical processes of eddy potential vorticity transport, momentum redistribution and jet formation in the Southern Ocean are studied with a view to understanding and parametrising their effects on the large scale circulation. Such a goal is desirable given the importance of the Southern Ocean in the world ocean circulation, and its role in the evolution of global climate. The study is conducted using a wind driven quasigeostrophic channel model. Without topography the flow adopts a configuration with an intense meandering central jet flanked by eddies. In the presence of a centrally located Gaussian topographic hill the flow configuration is drastically altered, taking on a double jet structure upstream of the hill and exhibiting a Rossby wave wake downstream of it. The mean mass transport is halved. The transient eddies are found to follow a simple closure hypothesis based on a diffusive parameterisation of of eddy potential vorticity transport in both topographic and flat bottomed cases provided that the dynamically irrelevant nondivergent part of the eddy potential vorticity flux is discarded in the topographic case. The standing and transient eddies are found to have distinct roles in the momentum balance, depending on the location in the channel, and the closeness to topography. In the presence of a zonally symmetric Gaussian ridge, the flow characteristics are again profoundly altered with a number of distinct zonal jets formed whose spacing is shown to be related to the topographic slope and the eddy energy of the flow. The phenomenon is observed in the Fine Resolution Antarctic Model.</p
Asymmetric response of European pressure and temperature anomalies to NAO positive and NAO negative winters
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Combining point correlation maps with self-organizing maps to investigate atmospheric teleconnection patterns in climate model data
No full textA new method for identifying teleconnection patterns in gridded climate data is presented. Point correlation maps constructed from NCEP/NCAR reanalysis sea level pressure (SLP) for the period 01.1984-12.2005 are used to train a self-organizing map (SOM), which topologically orders the patterns and provides a measure of frequency of pattern occurrence. Well known patterns can be identified within the SOM, such as the NAO, ENSO and the PNA, however the flexibility of the SOM allows these patterns to be viewed as part of a continuum of patterns, each identifiable as a variation within a defined teleconnection pattern. As the SOM is a non-linear method, asymmetries between patterns generated from opposite centres of action are revealed. Clustering the SOM patterns identifies the regions of the SOM corresponding to different teleconnection types by classifying similar patterns together. This retains the continuum of patterns, but allows generalization and characterization of the teleconnections present in the data. The patterns identified by the SOM can be used to evaluate the teleconnections in climate model SLP data. Point correlation maps are determined for the model data and compared to the SOM. By matching each of the NCEP/NCAR correlation maps and each of the model correlation maps with their most similar pattern on the SOM, discrepancies between the datasets are revealed. Additionally, the base points corresponding to the correlation maps for each teleconnection show the regions important to their existence. Differences in the location of the base points between NCEP/NCAR and the models provide insight into the biases underlying the model deviations from reality. The method can be extended to investigate other variables, for example the SOM can be trained using both SLP and geopotential height to investigate the 3D structure of teleconnections, while the location of the base points of the correlation maps for certain patterns can be used to assess the impact of teleconnections, such as rainfall and temperature patterns. <br/
Identifying Teleconnection Patterns from Point Correlation Maps using Self Organizing Maps
No full textTo identify atmospheric teleconnection patterns in 60 years of NCEP temperature, pressure and geopotential height anomalies, point correlation maps are presented to a Self Organizing Map (SOM), which topologically orders the patterns and provides a measure of frequency of pattern occurrence. Well known patterns can be identified within the SOM, such as the NAO, ENSO and the PNA, however the flexibility of the SOM allows these patterns to be viewed as part of a spectrum, or continuum, of patterns, each identifiable as a variation within a defined teleconnection pattern. The SOM patterns are then clustered to reduce the number of patterns and explore the separation of distinct patterns from the spectrum. Idealized periodic patterns of increasing complexity are used to test and explain the method.To assess the robustness of the method a SOM was constructed using point correlation maps for 60 years of NCEP surface temperature anomalies. Point correlation maps for the first and last 30 years are then compared to the SOM patterns constructed from the whole period. The patterns were robust and the pattern frequency data was able to identify the increased frequency of ENSO Modoki in the second half of the data, as observed in other studies, illustrating the method’s capability to detect changes within teleconnection patterns over time.This method can be extended by the use of correlation maps from multiple variables presented simultaneously to the SOM, helping to investigate the relationship between different aspects of the atmosphere. For example, correlation maps for surface temperature, surface pressure and geopotential height can be combined to evaluate the state of the atmosphere associated with specific patterns and how changes in the structure affect the form of the teleconnection patterns. Similar insights can be gained by using time lagged point correlation maps to investigate the predictability of teleconnection patterns
Influence of grazing formulations on the emergent properties of a complex ecosystem model in a global ocean general circulation model
No full textSensitivity to nonlinear equations may be a characteristic feature of biological models, particularly those that are complex. A complex marine ecosystem model (PlankTOM5.2) that incorporates multiple plankton functional types (PFTs) was embedded in a global ocean general circulation model (OGCM) and its performance assessed for four different formulations of multiple-prey zooplankton functional response: Michaelis–Menten (MM: Holling Type II), Sigmoidal (S: Holling Type III), Blackman (B) and Ivlev (Iv). Predictions of the four simulations were compared for the North Atlantic and North Pacific oceans. Remarkable differences were seen in both spatial extent and magnitude of predicted distributions of PFTs, as well as bulk properties, highlighting how the choice of functional response has a major impact on the resulting ecosystem structure. The range of average concentration of diatoms in surface waters was particularly marked, varying between 0.04 mg m?3 (B and MM) and 0.13 mg m?3 (S) in spring and between 0.01 mg m?3 (B) and 0.07 mg m?3 (S) in autumn. Differences in ecosystem structure affected predicted export flux, which varied by more than 25% among the simulations. Overall, our work highlights that accuracy is required in ecosystem formulation if reliable predictions are to be made when using complex marine ecosystem models embedded in OGCMs and therefore the need for further studies, with appropriate validation, that address structural sensitivity
The effect of ocean dynamics and orography on atmospheric storm tracks
No full textThe control of atmospheric storm tracks by ocean dynamics, orography, and their interaction is investigated using idealized experiments with a simplified coupled atmosphere-ocean climate model. The study focuses on the quasi-steady state for the storm tracks in the Northern Hemisphere winter mean. The experiments start with a background state without mountains and ocean dynamics, and in separate stages include orography and a dynamic ocean to obtain a more realistic control simulation. The separate effects of ocean dynamics, orography, and their nonlinear interaction are identified for the storm tracks and the surface thermodynamic forcing over the ocean.
The model study suggests that atmospheric storm tracks are a robust feature of the climate system, occurring at midlatitudes even if there is no orographic forcing or ocean heat transport. Ocean dynamics generally lead to a poleward shift in both the storm track and the maximum in atmospheric northward heat transport and induce a northeastward tilt over the Atlantic. This poleward shift is linked to the extra heat transport by the ocean and the tightening of sea surface temperature gradients on the western side of ocean basins. Orographic forcing causes along-track variability with a weakening of the storm track over the continents and induces a northeastward tilt over the western Pacific, which is associated with a stationary planetary wave train generated by the Tibetan Plateau. The interaction between ocean dynamics and orographic forcing plays a localized role, enhancing the contrast between the Atlantic and Pacific. Much of the response to the forcing is eddy mediated and transient eddies help to spread the influence of orographic and ocean forcing
A numerical model study of the effects of interannual timescale wave propagation on the predictability of the Atlantic meridional overturning circulation
No full textWe investigate processes leading to uncertainty in forecasts of the Atlantic meridional overturning circulation (AMOC). A climate model is used to supply initial conditions for ensemble simulations in which members initially have identical ocean states but perturbed atmosphere states. Baroclinic transports diverge on interannual timescales even though the ocean is not eddy-permitting. Interannual fluctuations of the model AMOC in the subtropical gyre are caused by westward propagating Rossby waves. Divergence of the predicted AMOC with time occurs because the waves develop different phases in different ensemble members predominantly due to differences in eastern boundary windstress curl. These windstress fluctuations communicate with interior ocean transports via modifications to the vertical velocity and the vortex stretching term dw/dz. Consequently, errors propagate westwards resulting in longer predictability times in the interior ocean compared with the eastern boundary. Another source of divergence is transport anomalies propagating along the Gulf Stream (and other boundary currents). The propagation mechanism seems to be predominantly advection by mean currents, and we show that the arrival of westward propagating waves can trigger development of these anomalies. The mean state of the AMOC has a small effect on interannual predictability in the subtropical gyre, most likely because eastern boundary windstress curl predictability is not strongly dependent on the state of the AMOC in the subtropics. Eastern boundary windstress curl was more predictable at 45{degree sign}N when the AMOC was in a strongly decreasing state, but, unlike at 30{degree sign}N, no mechanism was found linking windstress curl fluctuations with deep transports
NOC-MSM/PV_diffusion
No full textfirst release of the model data, figures and figure generating programs for journal article "Coefficient of diffusivity of potential vorticity and the eddy saturation regime" by Vladimir Ivchenko and Bablu Sinh
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